Such a tray is normally horizontally mounted in a vertical column, and the terms upward, downward, top, and bottom are used in the specification and in the claims with reference to this normal tray arrangement. The vertical column can for example be a column for counter-currently contacting gas and liquid, such as a fractionation column, wherein a plurality of similar trays are vertically stacked.
Each of such trays normally comprises a bubble area, which is an area on the tray plate that is provided with tray openings, e.g. perforations or special valves, through which gas can upwardly pass during normal operation. In this way gas is allowed to bubble through the liquid present on the tray during normal operation, thereby achieving intimate gas-liquid contact. Liquid is normally received on the tray from the liquid outlet of the downcomer of an upper tray. The tray is arranged such that liquid flows a certain distance over the tray during which time gas can bubble through, before the liquid is removed from the tray and guided to a lower tray via a downcomer.
The liquid outlets of the downcomers can in particular be arranged above the level of liquid on the liquid receiving tray. Such downcomers are commonly referred to as truncated downcomers.
U.S. Pat. No. 4,550,000 discloses a known tray which comprises a number of parallel rectangular sloped downcomers. A rectangular downcomer has a substantially rectangularly shaped inlet opening. A rectangular sloped downcomer is a downcomer of which the longer sidewalls are inclined towards each other in downward direction. The lower end of the known downcomers is formed by a bottom plate wherein the liquid outlet openings are arranged, so that interference of liquid streams from adjacent downcomers is prevented.
It was observed that a problem occurs when such downcomers are operated at high liquid loads. The liquid leaving through the liquid outlets falls with such a velocity on to the next lower tray, that the liquid may pass directly through any perforations directly below the downcomer. This is a problem since normally the liquid that is received on a certain tray must travel some distance to the next downcomer inlet opening so as to have sufficient residence time for gas-liquid contact on that tray. Also if trays with floating valves are used, the momentum of the liquid received on the lower tray may push the valves directly below the downcomers downwards, thereby closing the tray openings on the next lower tray, so that gas cannot bubble through the liquid receiving area. This problem is not encountered when no perforations or valves are arranged in a liquid receiving area underneath the downcomers of the upper tray. However this is undesirable since the unperforated liquid receiving area is arranged at the cost of bubble area, whereas it is normally desired to maximise the bubble area for optimum fractionation efficiency.
U.S. Pat. No. 4,550,000 discloses a solution to this problem by arranging a perforated structure made of expanded metal on the lower tray, underneath the liquid discharge openings of the upper downcomer. The perforated structure serves to break up and at least partly deflect the liquid stream. However, such perforated structures are not normally applied in practice because of their complexity and manufacturing cost.
Clearly the problem described hereinbefore increases with increasing velocity of the liquid impacting on the tray below. Particularly high liquid velocities are observed with trays comprising highly sloped rectangular downcomers as described in International patent application publication No. WO 01/60485.
U.S. Pat. No. 5,407,605 discloses a gas-liquid contacting tray comprising a perforated tray deck and a plurality of rectangular downcomers, each having a pair of planar sidewalls that are inclined towards each other so that the downcomers have triangular cross-section. The bottom part of the downcomers is sealed and the inclined walls are provided with liquid outlet openings in the form of perforations. In this way a horizontal velocity component away from the downcomer is imparted to the liquid exiting the liquid outlet openings. The known tray is useful for spreading the liquid over any packing which may be located below the tray. Also, when no packing is arranged so that the liquid is directly received by a lower tray, the horizontal velocity component helps directing the liquid onto the decking area of the tray below.
However, it was found that it is difficult to control or optimize the distribution of liquid from the downcomer outlet openings onto the tray below, in order to make optimum use of the bubble area. Under given process conditions in the column the flow path of liquid exiting from the known downcomer is mainly determined by the angle of the sidewall with the vertical.
UK patent application publication No. GB 2 002 650 discloses a vertical column comprising two trays that are vertically stacked in the column. On each tray a plurality of rectangular downcomers is arranged that extend parallel to each other over the width of the tray, in between perforated bubble area. The floor members of the downcomers are provided with openings through which the liquid exits vertically downwardly. In order to prevent weeping of liquid from the upper downcomers through the lower fluid contact area, transversely beneath the downcomer openings a lip means is arranged, that gives a horizontal velocity component to the liquid exiting the downcomer perpendicular to the length direction of the downcomer. In one embodiment of the known trays the downcomers on both trays extend parallel to each other, and the downcomers of the lower tray are transversely so much displaced, that the liquid from the upper tray is not discharged directly into the downcomer of the underlying tray. In another embodiment the downcomers on both trays extend in mutually perpendicular directions, and the liquid exiting from the upper downcomer flows parallel between the downcomer inlet openings on the lower tray.
It is desired to provide a gas-liquid contacting tray with a downcomer, which allows controlled or optimized distribution of the liquid from the downcomer over the tray below, in particular such that the use of bubble area on the tray below and the overall tray efficiency can be maximised.